One of several systems used to produce accurate, close-tolerance, precision holes, whether deep or relatively shallow, in metals, as well as other materials, is by use of a cutting tool known as a gun drill. Originally, gun drills were used to drill quite long holes through solid stock to be used as gun barrels. The dimensional tolerances and degree of straightness required for such holes were quite exacting, and with the advent of mass production, precision was required, i.e., the hole through each gun barrel was required, within very close tolerances, to be identical to all other holes in the lot of gun barrels being produced. The cutting tools originally developed to bore such holes in gun barrels have found much broader use, at present, and are used in a wide variety of situations where accurate, straight precision holes are required to be cut or bored.
Gun drills of various designs and modification are well known to those skilled in the art. Examples of various gun drill designs are described and illustrated in U.S. Pat. Nos. 534,009; 2,346,546; 2,418,021; 2,739,496; 3,153,356; 3,304,816; 3,320,833; 4,437,802; 4,804,300 and Re. 26,452. As Will be well understood by those skilled in the art, from a review of such U.S. Patents, gun drills are not inexpensive devices, as they require great skill and care in their production. Yet, gun drills are normally produced to a given set size, for boring a single diameter of hole with a specified set depth. To bore a hole of a different diameter and/or of a significantly different depth, another gun drill designed for that different hole must be used. Thus, those who practice gun drilling usually must stock a rather extensive variety of differing diametrical and depth-size gun drills, with a commensurate substantial amount of funding being invested in that stock.
Various attempts have been made to reduce the cost of gun drills in respect to maintaining the capability to bore a reasonably broad array of differing diametrical sizes of holes and/or a reasonably broad array of bore depths for each diametrical size of hole. For example, U.S. Pat. No. 3,304,816 discloses a multi-section tool, separable from its driving means, which uses a mating undercut arrangement in the form of a helix, on the corresponding male and female ends of the sections, to coaxially join the sections together, thus rendering the cutting tip detachable from the tool holder and providing means to vary bore hole diameters as well as a system for interpositioning of additional coaxial extensions to provide adjustable bore depth capability. Another example is shown in U.S. Pat. Nos. 3,153,356 and Re. 26,452 which disclose a gun drill arrangement in which the tool may be extended by threading sections coaxially together; the threads are helical in form with a specified helix angle range for locking the male and female threads to each other as the means for coaxially aligning the tool sections. Another example is disclosed in U.S. Pat. No. 2,346,546 which shows an arrangement for extending the length of the tool by slip fitting a formed male section into a correspondingly sized and shaped female section. Another example is provided in U.S. Pat. No. 534,009 which discloses a conceptually similar arrangement to that shown in U.S. Pat. No. 2,346,546, but with the corresponding male and female slip-fit sections being formed into interlocking axially aligned key and keyway sections. Another example is illustrated in U.S. Pat. No. 2,418,021 which shows and arrangement for extending the length of a gun drill, and for interchanging the cutting tip end, by means of a male pilot extension being slip fitted into a correspondingly sized axial bore in another section. Yet another example is illustrated in U.S. Pat. No. 3,320,833 which discloses means for interchanging gun drill cutting tips, and for extending the length of a gun drill, by axially bolting the corresponding pieces together. Even yet another example is provided by U.S. Pat. No. 2,739,496 which discloses a similar bolt arrangement, to that shown in U.S. Pat. No. 3,320,833, but with axially aligned corresponding and mating male-female conical surfaces. A final example is provided by U.S. Pat. No. 4,437,802 which discloses and arrangement of interchangeable, differing-sized cutting blades for gun drills.
It has been found that there is a critical requirement which must be met to provide the capability in a gun drill to bore close dimensional size tolerance holes, especially those that are relatively deep in relation to the diametrical sizing of the hole, while still maintaining exact straightness of those holes, as well as alignment of the holes exactly perpendicular to the axis of rotation of the tool, while providing precision, i.e., the ability to repeatedly bore exactly the same diametrical size of hole, time after time, all within a very close size tolerance, while keeping the respective holes all exactly straight, all exactly aligned parallel to each other and all exactly perpendicular to the axis of rotation of the tool. Simply stated, the requirement is that the overall axis of the tool must be the exact rotational axis of all contact surfaces of the tool, i.e., all surfaces of the tool, which come into direct contact with the work piece, must be exactly symmetrical with the overall axis of rotation of the tool. Of course, when the tool is a single piece, being machined all on the exact same centers, e.g., by way of a lathe and with finishing being accomplished on a cylindrical grinder and a tool grinder, the required axial alignment is not a significant problem. But when the tool is multi-pieced, e.g., where the cutting tips are replaceable and/or the tool is extendable by the addition of pieces or tool segments interposed along the rotational axis, problems in exact axis-of-rotation alignment, i.e., true symmetry of the multiple pieces one to the other, becomes a problem. Several of the foregoing published references mention and discuss this problem, but none seem to clearly and definitively disclose means to ensure a cure, or even to significantly diminish the problem.
The axial misalignment problem, outlined above, seems to be most acute in systems which seek to add axial extensions, either to change the diametrical sizing of the gun drill or to extend its length. Even though the components of the tool may initially be assembled in exact axial alignment with each other, it is extremely difficult to maintain that axial alignment during use of the tool. For example, the initial contact of the tool with the work piece causes a misaligned, asymmetrical and skewed vector of force on a point of the tool which is not on the axis of rotation. This vector of force tends to flex the tool even though the cutting tip may be held in position by a guide bushing, a common practice in gun drilling. The flexing results in some degree of bending and twisting along the linear section of the tool. Unless prevented, this bending and twisting will cause the adjoined tool sections to move in relation to each other, causing a misalignment of those sections and resulting in a disturbance of the symmetry of the work piece contact surfaces and the formation of a new and asymmetrical axis of rotation. Even a misalignment of symmetry, at a joint of the tool sections, of 0.0001-0.0002" may be compounded to a misalignment of 0.002-0.004" or more at the cutting tip, resulting in a bored hole which is oversized beyond the permitted tolerance range. Such a misalignment at the cutting tip, consequently, places a continuously greater relative stress on one side of the tool than the other, tending to cause the tool to wander from the intended straight path, resulting in a bore which is not exactly axially aligned with the axis of rotation of the tool; thus the work piece would have a misaligned hole not perpendicular to the original axis of rotation of the tool and not exactly straight over its length if the hole had any significant depth.
Flexing of the tool, resulting in relative movement of the adjoined sections at the joint therebetween, may also be caused by, for example, hard spots in the work piece, work-hardening of the work piece from heat build up, dulling of the edges of the cutting tip at an uneven rate, an uneven or disproportionate heat up rate of the different sections of the tool or compaction of metal chips at the cutting face and by breaking or chipping of the cutting tip. Whatever the reason for the flexing of the tool, if it is permitted to result in movement of the tool sections relative to each other, the accuracy and precision of the gun drilling operation is lost, tolerances must be increased, and the cost of producing exactly axially aligned, exactly straight and exact diametrically sized bores, on a repeatable basis, are substantially increased due to the need to use secondary finishing operations.
None of the above cited published references appear to provide adequate means to ensure that there is no movement of adjoining tool sections, relative to each other, and thus potential misalignment of the respective axes of rotation is a significant risk. Further, none of the above cited published references appear to provide means by which adjoining tool sections can be exactly axially aligned on a single true axis of rotation, upon assembly, without first removing the tool holder from the machine tool, then tediously assembling the tool section with the tool holder on centers, by use of sensitive dial indicators, then re-mounting the assembly to the machine tool (the latter step which may, in some cases, cause misalignment of the tool holder with the axis of rotation of the machine tool spindle).